This present invention relates to the art of electroplating and more particularly to a method for producing electrodeposits of copper particularly useful for manufacturing printed circuits.
This invention is applicable to the formation of both conventional electrodeposited copper foil, as well as so called "ultra-thin" electrodeposited foils of copper on an aluminum carrier. While the present invention will be primarily described in the context of conventional unsupported foils, it is not intended in any way to be limited to such foils, but rather to also include foils of copper on an aluminum or other carrier. These unsupported foils may have a thickness on the order of from about 12 to about 35 microns or more, while so called supported foils are usually on the order of from about 5 to 12.0 microns and are typically deposited on an aluminum sheet of 50 to 75 microns. For the purposes of this disclosure, the term support is intended to include any suitable substrate, while the term aluminum is intended to embrace the commercially pure metal as well as alloys of the metal which are predominantly aluminum.
The well known basic techniques for manufacturing printed circuit boards involves depositing copper on a revolving drum or on a temporary carrier such as sheet of aluminum; optionally applying a treatment coating to the exposed surface of the copper foil (such as taught for example in U.S. Pat. No. 3,585,010); applying the exposed or treated surface of the copper to a printed circuit board, such as an epoxy resin impregnated fiberglass mat or substrate; bonding the copper surface to the epoxy resin through the use of heat and pressure, and then removing the temporary carrier if any.
In order for this laminate of copper foil on the resinous substrate to yield a quality printed circuit board, among other properties the foil must be highly pore-free and securely bondable to the substrate, this is particularly critical with thin foils. One way to improve the bond between the copper foil and the substrate is to produce a nodularized exposed surface on the copper foil, such as by producing a dentritic outer surface.
It should be noted that in the industry the term dendrite and/or dendritic deposit etc. have both a broad and a specific meaning. The specific meaning relates to deposits having thin needle like and/or tree like structures which tend to be very fragile and require an overlay of basic copper to assure good adhesion. The broad meaning of these terms relates to laying down of a roughened surface generally, including the production of a nodular or bumpy surface which adheres well without need for an overlay.
In the prior art, as first disclosed in U.S. Pat. No. 3,293,109 and later adapted in U.S. Pat. No. 3,990,926, a two or more step electrodeposition of copper had been necessary in order to produce a foil highly pore-free and securely bondable to the resinous substrate. Typically, this involves a first copper electroplate to build up a thickness of up to about 50 microinches to ensure a uniform copper substrate base and then at least one more bath and/or a different current density to provide the greater thickness buildup as needed and to nodularize the outer copper surface for increasing the bond strength between the foil and the substrate to which is it bonded or laminated.
Thus, a multi-step process of electroplating copper has heretofore been needed to yield a highly pore-free foil which is securely bondable to the resinous substrate or, in other terms has a high "peel strength". "Peel strength" is a conventionally used term to refer to the strength of the bond between the foil and the resinous substrate. Peel strength in excess of about 7 lbs./in., according to the standardized measuring method ASTM D/1867 is generally deemed necessary to satisfy printed circuit requirements.
The multi-step process although capable of producing pore-free foil with a nodularized outer surface does have the drawback of requiring close control and regulation between the steps. Not only does each step need careful monitoring but also process variables of each step such as bath composition, current density in the bath, temperature, etc. must be carefully coordinated with those of each other step. For example, if a two-step process is used in which the bath composition is changed in the second step, close coordination is needed between bath composition and other variables in the first step with the new bath composition of the second step. These control and coordination requirements do not yield a simple process. Even with careful control of this multi-step process, its complexity often gives rise to reliability problems. Additionally, the multiplicity of steps would give rise to the need for more space and equipment and corresponding expense associated with them.
U.S. Pat. No. 3,857,681 to Yates taught the application of a continuous high current density from an auxillary anode positioned beyond the end of the primary anode at exit end of the bath, i.e. posterior to the primary anode in the process, to provide a nodular or dendritic deposit. In practice, however, the Yates techniques provided a powdery deposit having very poor adhesion to the base copper deposit. In later U.S. Pat. No. 4,490,218, Kadija et al. teach the use of a plurality of high current density auxillary anodes embedded in the primary anode. While the current to the auxillary anodes may be pulsed, the surrounding lower current density is continuous. According to Kadija, their high current density anodes produce dendritic structures while the primary anode provides an overlay of base copper to bond the dendrites to the underlying base copper deposit.
Accordingly, it is an object of the present invention to provide an improved process of electroplating foil.
Another object of the present invention is to provide a one-step copper electroplating process which yields a uniform, virtually pore-free copper foil with a nodularized surface for strong adherence to an epoxy resin impregnated fiberglass circuit board.
Yet another object of the present invention is to provide a process which increases initial copper nucleation and provides a nodularized outer surface.
Yet another object of the present invention is to provide an improved process for electrodeposition of copper on a carrier.
Yet still another object of the present invention is to provide an improved process for electroplating pore-free copper on an aluminum carrier with a nodularized surface for strong adherence to an epoxy resin impregnated fiberglass circuit board.
Other objects and advantages of the present invention will become apparent from the following detailed description thereof, which includes the best mode contemplated for practicing the invention.